1. Field of the Invention
This invention relates to a color processing method and a color processing apparatus for transforming the color signal in the color space such as L*a*b*, and RGB to the color signal of four colors including black, and relates to a recording medium that stores a program for implementing such color processing method, and an image marking apparatus on which such a color processing apparatus is mounted.
2. Description of the Related Art
When a color image is color-printed by means of xerography, four-color printing by yellow (Y), magenta (M), cyan (C), and black (K) is used usually. On the other hand, the general color signal is the color signal on the three-dimensional color space such as L*a*b*, L*u*v* color spaces that are independent of the device or RGB color space that is used for display signal. Therefore, when a color image is to be color-printed, the color signal on the three-dimensional color space must be transformed to the four-dimensional color space. However, because the transformation is performed between different dimensions, one-to-one relation is not held, and there exist plural combinations of the color signal on the three-dimensional color space and the four-color signal for reproducing the color signal on the three-dimensional color space. Hereinafter, the amounts of yellow, magenta, cyan and black components are referred to as Y, M, C and K, respectively.
To determine the combination of the color signal on the three-dimensional color space and the color signal on the four-dimensional color space, there have been known a method in which Y, M, and C are calculated, the under color is removed, and K is added, and also a method in which K is determined at first by a certain method and Y, M, and C are then determined corresponding to K. Recently, the latter method has been used popularly in view of the excellent color reproduction, and other various methods are tried.
For example, in the method described in Japanese Published Unexamined Patent Application No. Hei 5-292306, at first, an achromatic K for reproducing the input color signal under the condition of Y=0%, M=0%, or C=0% is weighted as set previously, and a new K is determined. Based on the new K, Y, M, and C are determined so as to reproduce the input color signal. Thereby, it is possible to control K corresponding to the purpose and it is also possible to implement high accuracy color reproduction simultaneously.
Furthermore, in the method described in Japanese Published Unexamined Patent Application No. Hei 6-242523, at first, a maximum K of the four-color signal that reproduces the input color signal under the condition of Y=0%, M=0%, C=0%, or K=100% is calculated. Then, the minimum K of the four-color signal that reproduces the input color signal under the condition of Y=100%, M=100%, C=100%, or K=0% is calculated. Thereafter, a new K is determined by use of a parameter that has been set on the intermediate point between the maximum K and the minimum K calculated as described above, and Y, M, and C that reproduces the input color signal is determined based on the new K. Thereby, the four-color gamut that is a reproducible color gamut with four colors including black can be used ultimately.
A coverage restriction is imposed to general output devices. The coverage restriction means the upper limit on the total amount of recording material such as toner or ink that is used to reproduce the color signal. The coverage restriction is applied to prevent the reduced reproducibility or high impasto print due to excessive use of recording material such as toner or ink, or to protect an output device.
However, the above-mentioned conventional methods do not consider the coverage restriction. Therefore, when K is controlled according to the parameter that has been previously set, the reproduction could not be implemented though that is in the reproducible color gamut. In other words, because the calculated Y, M, C, and K is outside the range of from 0% to 100%, the color gamut compression is caused resultantly, and the color reproduction accuracy becomes poor.
FIG. 14 is a graph for describing an example of the relation between the lightness and K in the conventional method. FIG. 14 shows K at the color different in the lightness with a certain chroma and hue. In FIG. 14, the abscissa represents L* (lightness) and the ordinate represents K, and an exemplary locus of the maximum K to L* and an exemplary locus of the minimum K are shown with a bold solid line and a bold broken line respectively. At that time, in the case of the method described in Japanese Published Unexamined Patent Application No. Hei 6-242523, K can be controlled in the range enclosed by a bold solid line and a bold broken line. In this example, the lightness range of L*≧B is given as the color reproduction area.
However, usually the coverage restriction is applied inevitably. In FIG. 14, the border lightness that is reproducible with four colors and satisfies the coverage restriction is shown with a fine line parallel to the ordinate (L*=A). Because of the coverage restriction as described hereinabove, Y, M, and C are outside the range of from 0% to 100% in the case where the K in the hatched range is used, and the reproduction becomes impossible. In particular, though the L*a*b*(L*≧A) in the hatched range is reproducible correctly, the reproduction is impossible because K is not determined properly.
Furthermore, the portion where the total coverage (sum of Y, M, C and K) is largest locates on the lower periphery for both lightness and chroma in the three-color gamut that is reproducible color gamut with three colors except black, and it is likely that the total coverage on the periphery exceeds the coverage restriction. However, in the case where Y, M, and C are replaced with K to satisfy the coverage restriction, the proper K cannot be calculated by the conventional method.
The present invention has been accomplished in view of the above-mentioned problem, and provides a color processing method and a color processing apparatus that are capable of calculating a proper K with consideration of the coverage restriction to thereby improve the color reproduction accuracy when the four-color signal including the black component is generated from the input color signal in the input color space, and are capable of controlling the preferable K correspondingly to the lightness, chroma, and hue by controlling K by use of the parameter that has been set previously, provides a recording medium that stores a program or the like for implementing such a color processing method, and provides an image marking apparatus on which such a color processing apparatus is mounted.